Sean Parker has joined Twitter -- not in an executive capacity, but as just a regular ol' tweeter.

Parker -- who co-founded Napster and later served as president of Facebook -- sent out his first tweet Monday. In it, he apologized to Facebook CEO Mark Zuckerberg and gave credit to Scooter Braun, a talent manager whose clients include Justin Bieber, for getting him to join the social-networking site.

Parker, who was memorably portrayed by Justin Timberlake in the Facebook movie The Social Network as a partying playboy, last week revealed that he is reuniting with Napster's Shawn Fanning to create a live video site dubbed Airtime. He also is backing Spotify and has a Facebook charity application titled Causes.

Meanwhile, Napster said earlier Monday that it will merge with competing music subscription service Rhapsody.